This invention relates to electronic apparatus and particularly to transistorized voltage regulators.
Circuits are known that use a transistor for producing a regulated direct output voltage. A simple, yet representative, form of such a circuit is shown in FIG. 1, where a single transistor Q1 of NPN polarity has its collector connected through a resistor R1 to an input terminal and its emitter connected to an output terminal. The base of Q1 is connected through a resistor R2 to the input terminal and also through zener diode CR1 to a common line. The circuit contemplates a positive going voltage at the input terminal as compared with the common line. Assume the source voltage at the input terminal ramps up at a gradual rate, typically linearly, to a level that exceeds the desired output voltage regulation level.
FIG. 4 is a plot of direct voltage with time where line A shows the increasing source voltage, while dashed line B is the desired output voltage regulation level. As the source voltage increases from zero, the output voltage of the circuit of FIG. 1 will follow the DC input voltage until the regulation level is reached, as shown by line C of FIG. 4. The reason for this is that the transistor Q1 of FIG. 1 is supplied base drive continually, and continually produces an output following the rate of increase of the input up to the level at which it is limited by the voltage drop of the zener diode. Numerous variations of the basic circuit shown in FIG. 1 have been employed. Generally, these have to do with additional elements for maintaining the regulated voltage level more precisely and to provide current limiting protection for a load connected at the output terminal.
Such circuits as that of FIG. 1 and its variations which have the characteristic illustrated in line C of FIG. 4 are very useful in many applications. There are applications, however, where the provision of a voltage regulator producing output voltage at a gradually increasing rate in accordance with a ramped source is disadvantageous. FIG. 5 shows one such application. In FIG. 5 is shown a circuit for an electromechanical switching device for a circuit breaker where the mechanical breaker contacts 10 associated with a three-phase line are controlled to produce a change of state from on to off, or the reverse, by energization of a coil 12 associated with a movable core 13. The coil has a switching device 14, such as a transistor, connected with it which is controlled by a logic circuit 16, referred to in the art as a high threshold logic (HTL) circuit. This is a known form of logic circuit offering advantages by reason of high noise immunity and moderate power dissipation. For the HTL circuit to work effectively, it is necessary that the voltage supplied to it be one that has an abrupt change between a zero level and the regulated voltage level. If not, then the logic circuit can see less than its rated supply voltage, typically 15 volts for HTL, which can result in confusion of the logic states. The HTL circuit may receive inputs from a variety of sources, such as transducers that monitor various conditions appearing on the power buses. In order to guarantee the output states of the logic circuit, the supply voltage to it from the voltage regulator 18 should be substantially free of voltages at magnitudes intermediate between zero and the rated supply voltage. Therefore, for applications such as that of FIG. 5, the voltage regulator should not be one like that shown in FIG. 1, where the output of the regulator gradually increases from zero to the regulation level.
The apparatus schematically shown in FIG. 5 is that generally referred to as an electrical load control unit or ELCU, as used, for example, in aircraft electrical systems. It will be apparent that the conditions imposed upon the voltage regulator in this example may obtain in other cases where regulated voltage sources are required.
In the past, in order to achieve the required voltage regulation characteristic for applications such as FIG. 5, there have been used voltage regulators such as that illustrated in FIG. 2. Here, the components Q1, CR1, R1 and R2 are connected in the same manner as the correspondingly numbered components of the circuit of FIG. 1. However, in addition, this circuit utilizes transistors Q2 and Q3, of the same polarity as Q1, and associated resistors R3, R4, and R5 and zener diode CR2, connected in the manner shown to serve as a clamping circuit to clamp the voltage across the zener diode CR1 to zero until the input voltage reaches the desired level which is set by the voltage drop of zener diode CR2 and the base-emitter voltage of transistor Q3. At this voltage level, the clamp is effectively removed and the regulator operates normally at the regulation level. The additional components of FIG. 2 alter the performance of the circuit as compared with that of FIG. 1 so that now the output characteristic is as shown by curve D of FIG. 4 such that the output voltage is kept at the zero level until, or almost until, the source voltage reaches the regulation level. At this point, the output voltage makes a marked change in a very brief time from zero up to the regulation level. This is the desirable characteristic of a voltage regulator for applications such as that shown in FIG. 5 and is the type of characteristic that is sought by the voltage regulator of the present invention. However, the circuit of FIG. 2 is made complex and expensive by the additional elements Q2, Q3, R3, R4, R5 and CR2 so that it is therefore desired that essentially the same regulation characteristics as is exhibited by the circuit of FIG. 2 be provided in a circuit of greater simplicity and economy, and this is the principal object of the present invention.